A photonic network including a plurality of optical nodes and optical fibers that connect the optical nodes is conventionally known. FIG. 4 is a diagram illustrating a configuration of a photonic network. When a client signal is transferred through the photonic network, an optical node transmitting a signal converts the client signal to an optical signal, and transfers the converted optical signal to an optical node receiving the optical signal from the transmitting optical node, and the receiving optical node converts the optical signal to the client signal. A path from the transmitting node to the receiving node through which the optical signal passes is referred to as an optical bus.
In the photonic network illustrated in FIG. 4, an ROADM (Reconfigurable Optical Add/Drop Multiplexer) node is used for the optical node. The ROADM node is a node capable of performing branching and insertion of the optical signal in units of wavelengths from a wavelength multiplexing (WDM: Wavelength Division Multiplexing) signal transferred from an adjacent ROADM node, and transferring the WDM signal to the adjacent ROADM node. A wavelength selection switch (e.g., see Patent Document 1 has recently been used for the ROADM node.
A wavelength selection switch that has been put to practical use, for example, includes nine input ports and one output port, and an interface that can set an optical signal output from the output port to any wavelength from any input port. Further, the wavelength selection switch has a function of setting an optical attenuation factor for each wavelength in order to equalize levels of the optical signals from the optical output ports. FIG. 5 is a diagram illustrating a configuration of an ROADM node including a wavelength selection switch. In FIG. 5, the ROADM node includes a wavelength selection switch (WSS) that switches a path of an optical signal, an optical amplifier, a transmitter that converts a client signal into an optical signal and transmits the optical signal, a receiver that receives the optical signal and converts the optical signal to a client signal, and an optical coupler.
In an example, the wavelength selection switch includes a wavelength separation element such as a diffraction grating that separates a path of a wavelength of a WDM signal from each port, and a number of movable mirrors that change the path of each wavelength. Movement of the mirror is realized through application of a voltage.
In this configuration, the wavelength selection switch needs to stably realize switching of an optical signal path in a steady state and attenuation factor adjustment for reduction of a level deviation between wavelengths at a wide range of environmental temperatures. Therefore, a control system having, for example, an external interface function for receiving control information of a relatively complicated control circuit having a temperature compensation function or the like and a wavelength selection switch, and a monitoring function of checking normality of an operation thereof is often built in the wavelength selection switch.
FIG. 6 is a block diagram illustrating a configuration of a wavelength selection switch system. In the wavelength selection switch system 1 illustrated in FIG. 6, an interface with the outside is a digital interface, and a wavelength selection switch system 1 includes mainly a digital circuit. Since driving of a minor 21 in a wavelength selection switch 2 is performed through voltage control, a digital-analog conversion circuit (DAC) 32 is connected to the minor 21 and an analog-digital conversion circuit (ADC) 33 is connected to a sensor part such as a temperature sensor 22 for temperature compensation, but all parts other than these are configured as digital circuits. In the control unit 3 in the wavelength selection switch system 1, a control circuit 31 including, for example, an FPGA (Field-Programmable Gate Array) provides instructions to calculate a driving voltage of the minor 21 based on configuration information of the minor 21 held in a nonvolatile memory 36 and a temperature of the wavelength selection switch from the temperature sensor 22 and to output the voltage to the DAC 32 when a supply voltage is supplied.
For example, in a structure in which a mirror 21 for 100 wavelengths is included and control is performed with three electrodes per mirror, a DAC (or a plurality of DAC groups) including 300 (100×3) output ports is used. In a steady state, processing operations (1) to (3) are repeated in which the control circuit 31 (1) reads mirror configuration information in the nonvolatile memory 36, (2) calculates a temperature compensation value using temperature information from the temperature sensor 22, and (3) instructs the DAC 32 to set a voltage. Further, the control circuit 31 performs rewriting of the nonvolatile memory 36 upon receiving a switch configuration instruction from an I/O port via an input and output interface 34, and controls an operation of the minor 21 based on rewritten mirror configuration information after the rewriting.